1. Field of the Invention
The present invention generally relates to solid state relays, and more particularly to a galvanic isolated solid state relay having output-side feedback.
2. The Prior Arts
A solid state relay (SSR) is a switch device made by solid state electronic components. SSRs utilize the on/off switching characteristics of semiconductor components such as triodes and triacs to open and close a circuit without mechanical contacts and without causing sparks. SSRs are therefore referred to as contactless relays. In contrast to the electro-mechanical relays (EMRs), SSRs do not rely on the physical actuation of mechanical parts and, therefore, have numerous advantages such as small form factor, fast response, high reliability, long operation life (e.g., a SSR can be turned on/off up to 108˜109 times, which is at least a hundred folds better than an EMR), low noise and surge interference, more robust to shocks and impacts, just to name a few. In addition, a SSR can be driven by a low-powered signal to control a very large AC current. A SSR therefore can be safely and reliably employed at the output interface of a digital system.
A conventional SSR 1, as shown in FIG. 1, has a pair of system-side terminals for connecting to a digital system and a pair of output-side terminals for connecting to the external system or load under control. The SSR 1 uses a photo coupler as a galvanic isolation device to separate the system side and the output side, but allow the digital control signal from the system side to be transmitted to the output side. Based on the ON/OFF digital control signals from the digital system, a driving circuit 12 at the system side drives a light emitting device (i.e., the light emitting diode 11) of the photo coupler 10 to emitted light signals (represented by the arrow heads). A light sensing device (i.e., the photo transistor 13) at the output side of the photo coupler 10 detects the light signals and produces electrical signals corresponding to the ON/OFF digital control signal to drive the control circuit 14, which in turn controls the external load or system (i.e., the load 16).
The galvanic isolation device is, as a matter of fact, a kind of energy conversion device. In addition to the foregoing photovoltaic devices (i.e., photo couplers), other possible means include thermoelectric devices which convert between thermal energy and electrical energy, piezoelectric devices which convert between pressure and electrical energy, and electromagnetic device which convert between magnetic energy and electrical energy. One of the most common galvanic electromagnetic conversion devices is the transformer.
The conventional SSR has a number of disadvantages. For example, the output side requires the configuration of another galvanic isolated power source 17, in addition to the power source 15 at the system side. Besides, the control circuit at the output side could only function either as a current source or as a current sink to the load, meaning a less flexible applicability for the conventional SSR.
The foregoing conventional SSRs have another major flaw which limits the control signal conversion in one direction from the system side to the output side only. Due to the galvanic isolation between the system side and the output side, the digital system cannot obtain the status of the output side, such as whether the external load or system under control responds correctly to the control signals. To achieve such a goal, a feedback circuit comprising another photo coupler, transformer, or similar galvanic isolation device is required, so that the electrical output status of the output side such as its voltage, current, frequency, and whether it is in a faulty condition like open-circuited, short-circuited, or overloaded, could be fed back to the system side for the digital system's monitoring. Further more, the digital system would also require the configuration of additional input ports for connecting to the feedback circuit. All of the above increase the cost and wiring difficulty of the SSR and the digital system.